Introduction

Many keyboards include some "special keys" (also called hotkeys, such as HP Quickplay), which are supposed to execute an application or print special characters (not included in the standard national keymaps). The lack of specification for these extra keys makes it impossible for the kernel to know what to do with them and that's why we need to map the keys to actions. There are two ways of doing that:

The universal way using Xorg utilities (and eventually your desktop environment tools)

The quicker way using a third-party program to do everything in GUI, such as the Gnome Control Center or Keytouch

Before starting you need to learn some vocabulary ...

A scancode is the lowest identification number for a key. If a key doesn't have a scancode then we can't do anything because it means that the kernel doesn't see it.

A keycode is the second level of identification for a key, a keycode corresponds to a function.

A symbol is the third level of identification for a key, it is the way Xorg refers to keys.

Step 1: Check for keycodes

Most of your keys should already have a keycode, or at least a scancode. Keys without a scancode are not recognized by the kernel.

The diagnosis

Using xev

Use the graphical X program "xev" (without having to switch to a console environment).
Install the xev program:

pacman -S xev

With the following line you can start xev and directly grep the important parts:

In the example below I pressed the "a", "r", "c" and "h" keys and two of the media keys on my Dell keyboard. This gives me the following output:

38 a
27 r
54 c
43 h
153 NoSymbol
144 NoSymbol

This means that the "a", "r", "c" and "h" keys have the keycodes 38, 27, 54 and 43 and are properly bound while the media keys with the keycodes 153 and 144 have no function yet, which is indicated by "NoSymbol". If you press a key and nothing appears in the terminal, this means that the kernel doesn't see that key or that it is not mapped.

Using showkey

The universal way to know if a key has a keycode is to use the kernel showkey program. showkey waits for a key to be pressed and if none is during 10 seconds it quits, note that this is the only way to exit the program. To execute showkey you need to be in a real console, it means not in a graphical environment so switch using Ctrl+Alt+F1.

$ showkey

and try to push your hotkeys. If a keycode appears the key is mapped, if not it can mean either that the kernel doesn't see the key or that the key is not mapped.

2.6 kernels

According to the keymap man page:

Note: In 2.6 kernels raw mode, or scancode mode, is not very raw at all. Scan codes are first translated to key codes, and when scancodes are desired the key codes are translated back...there is no guarantee at all that the final result corresponds to what the keyboard hardware did send.

This is relevant if the keymaps obtained from showkey and the ones set by setkeycodes differ from the ones obtained by xev in X. Keep this in mind when translating the keymaps into keysyms using xmodmap (See Extra Keyboard Keys in Xorg).

Conclusion

If all your keys have a keycode you can go directly to the second step.

If not keep reading below:

Know if a key has a scancode

If a key doesn't have a keycode you can know if it has a scancode by looking at the kernel log using the dmesg command:

$ dmesg|tail -5

If when you press the key something like that appears:

atkbd.c: Unknown key pressed (translated set 2, code 0xf1 on isa0060/serio0).
atkbd.c: Use 'setkeycodes e071 <keycode>' to make it known.

If nothing new appears in dmesg then your key doesn't have a scancode, which means that it is not recognized by the kernel and cannot be used.

Step 2: Map keycodes

In Console

When we are in console, we can use our hotkeys to print a certain character. Moreover we can also print a sequence of characters and some escape sequences. Thus, if we print the sequence of characters constituting a command and afterwards an escape character for a new line, that command will be executed!

In Xorg

When we are in a graphical environment we may want a key to print a special character or execute a command. There are many ways of doing that and they are covered in a dedicated article: Extra Keyboard Keys in Xorg.

Laptops

Asus M series

In order to have control over the light sensor and the multimedia keys on your Asus machine, you will have to open your terminal as root and open the next file with your favorite text editor:

/etc/rc.local

and then add the next line of code at the bottom of the file:

$ echo 0 > /sys/devices/platform/asus-laptop/ls_switch

Note that this may work for other Asus notebook models.
See the detailed article: lineak.